1. Field of the Invention
This application is related to integrated circuits and, more particularly, to amplifier circuits.
2. Description of the Related Art
In general, a telescopic operational amplifier (i.e., cascode operational amplifier or telescopic cascode operational amplifier) is a single-stage operational amplifier including a differential input stage, a load, a cascode circuit including a pair of cascode devices coupled between the input stage and the output node, and another cascode circuit including a pair of cascode devices coupled between the output node and the load. Typically, a bias circuit supplies DC operating current to the amplifier. Telescopic operational amplifiers are used in a variety of mixed-signal applications, including phase-locked loops. Advantages of telescopic operational amplifiers over other operational amplifier topologies (e.g., operational transconductance amplifiers, folded cascode amplifiers, and multi-stage amplifiers) include superior stability, lower noise, and higher gain achievable in a single gain stage. However, telescopic operational amplifiers exhibit inferior input and output common-mode range as compared to other operational amplifier topologies. Those shortcomings typically do not affect amplifiers with a large enough power supply voltage and/or applications having enough voltage headroom (i.e., where the input or output voltage swing of the telescopic amplifier is relatively small as compared to the power supply voltage). As integrated circuit manufacturing processes advance to use finer geometries, power supply voltages are decreased to obtain reliable performance from CMOS devices. Accordingly, integrated circuit designers substitute conventional telescopic operational amplifiers with other amplifier topologies that have greater common-mode range than telescopic operational amplifiers, but are inferior to telescopic operational amplifiers in other aspects of performance.